Dual stage optical isolator with reduced polarization mode dispersion and beam offset

Abstract

A double-stage (multi-stage) optical isolator is composed of two pairs of birefringent wedges (polarizers) with a Faraday rotator sandwiched by a pair of the wedges. The wedge-shaped polarizers, each defining a wedge angle, are disposed such that the wedges of the third and fourth polarizer (second stage) are arranged generally opposite relative to the wedges of the first and second polarizer (first stage) respectively, the wedge angles and the angles of incidence in the forward direction selected such as to minimize offset between the line of propagation of an optical beam passing through the isolator in the forward direction. The isolator has a PMD compensation element disposed between the first and the second stage to minimize polarization mode dispersion of light beams propagating through the isolator. The isolator has a very low PDL, virtually no PMD, high isolation and virtually no displacement (offset) between the input beam and out beam line of propagation.

1 . A multi-stage optical isolator having at least two stages, each stage comprising two birefringent wedge polarizers and a rotator therebetween, 
 the first stage comprising a first birefringent wedge polarizer for splitting an input beam into two orthogonally polarized sub-beams, a first rotating means for rotating the polarization of the two sub-beams received from the first polarizer and a second birefringent wedge polarizer disposed in the optical path of the sub-beams from the first rotating means,    the second stage comprising a third birefringent wedge polarizer disposed in the optical path of beams from the second polarizer, a second rotating means for rotating the polarization of received from the third polarizer, and a fourth birefringent wedge polarizer disposed in the optical path of beams from the second rotating means,    the input beam defining an input line of travel and light output from the fourth polarizer defining an output line of travel in a forward direction,    wherein the wedge polarizers of the second stage are disposed such that the wedges of the third and fourth polarizer are arranged generally opposite relative to the wedges of the first and second polarizer respectively, and the angles of incidence in the forward direction and the wedge angles selected such as to minimize offset between the input line of travel and the output line of travel.    
     
     
         2 . The multi-stage optical isolator of  claim 1 , wherein the angle of incidence of the input beam on the first polarizer is of opposite sign relative to the angle of incidence of any light beam incident on the third polarizer in the forward direction.  
     
     
         3 . The multi-stage optical isolator of  claim 1  having an even number of stages, more than two, the arrangement of a second and any subsequent pair of stages and the respective angles of incidence following the same relationship to the preceding stage as in said first and second stage.  
     
     
         4 . The multi-stage isolator of  claim 1  comprising a PMD compensation element disposed between the first and the second stages in the optical path of beams propagating between said stages, the properties of the element selected to minimize polarization mode dispersion of light beams propagating through the isolator.  
     
     
         5 . The isolator of  claim 4  wherein the optical axes of the first and fourth polarizers are of substantially the same magnitude and identically oriented and the optical axes of the second and third polarizers are of substantially the same magnitude and identically oriented.  
     
     
         6 . The isolator of  claim 4  where the optical axes of the crystals forming the first, second, third and fourth polarizer are the same.  
     
     
         7 . The isolator of  claim 1  wherein the rotating elements are Faraday rotators controlled each by a magnetic field, the direction of the magnetic field for the Faraday rotators selected to correspond to the optical axis orientation of the wedge polarizers for recombining the two sub-beams in the forward directions and separating the sub-beams in the reverse direction.  
     
     
         8 . The isolator of  claim 1  further comprising a reciprocal rotating element disposed in the optical path of the sub-beams between the polarizers of the second stage and selected for allowing magnetic field direction for the first stage Faraday rotator and the second stage Faraday rotator to be the same.